Sidereal Time

On the back, the watch turns its face to the stars and the dial reveals all the beauty of the night sky. The heavenly vision depicted on a dedicated sky reveals the constellations that are visible from a given latitude.
This second face of time is inextricably linked with the path of the planets and the stars within the infinity of the cosmos, which is why the back of the swivel case is entirely dedicated to the essential astronomical realities of the universe, of which the starry sky is an integral part.
A disc in an oval aperture in the dial, adjusted for the northern or southern hemisphere depending on where the watch’s owner lives, represents the angular displacement of the distant celestial bodies and the constellations. This is no easy task, since sidereal time has its own particularities. The sidereal day is the time that elapses between two successive transits from the vernal equinox point to the meridian from the place of observation. The vernal equinox point is in fact an imaginary point which cannot be observed directly and which corresponds to the beginning of spring.
Expressed another way, the sidereal day is almost equal to the time necessary for the Earth to make a complete revolution about its axis. To be precise, the sidereal day is 3 minutes 56 seconds shorter than the mean solar day. All the same, it is also divided into 24 (sidereal) hours. Expressed mathematically, that means that a sidereal second lasts exactly 0.99726957 of a solar second. These are details that Jaeger-LeCoultre’s watchmakers could hardly ignore. And that’s why the Reverso grande complication à triptyque contains a mechanism designed to transpose the mean solar time into sidereal time. The disc of the celestial bodies and constellations therefore completes a turn every 23 hours, 56 minutes and 4 seconds (Civil Time).